U2F (#1304)

* Delete U2F tokens alongside WebAuthn

* Bring back u2f apis

src/Core/Models/Business/U2fRegistration.cs

@@ -0,0 +1,9 @@
+namespace Bit.Core.Models.Business
+{
+    public class U2fRegistration
+    {
+        public string AppId { get; set; }
+        public string Challenge { get; set; }
+        public string Version { get; set; }
+    }
+}